It has conventionally been well known that compositions containing silicones as the major component are used to produce pressure-sensitive tapes and various kinds of labels. In these processes, the compositions are applied on surfaces of substrates such as various kinds of papers, synthetic films, and fabrics, and cured to form films of the compositions, thereby imparting non-adhesive properties, i.e., release properties, which prevent or reduce adhesion of tacky substances. Such silicone compositions which impart release properties are required to have properties such as coating property and curability for film formation, and cured films obtained from the compositions are required to have properties such as release properties and the property of not adversely affecting pressure-sensitive adhesive layers (the composition component's property of not transferring to pressure-sensitive adhesive layers; this property is evaluated, for example, in terms of percentage of subsequent adhesion for the pressure-sensitive adhesive layers). In particular, the release properties of cured non-adhesive films are important for the specifications of pressure-sensitive adhesive tapes and various labels, so that the silicone compositions are required to give various degrees of peel strength ranging from light release to heavy release. Pressure-sensitive tapes or the like are roughly classified in terms of peel strength into the following three groups, although measured peel strength values vary depending on the measuring methods and other conditions: those having peel strength of 30 g/5 cm or less are light-release; those having peel strength more than 30 g/5 cm but not more than 100 g/5 cm are medium-release; and those having peel strength exceeding 100 g/5 cm are heavy-release. Such regulated peel strength are required not to change with the lapse of time and to be maintained stably. This long-term stability of peel strength is important when medium-to-heavy release is required.
Known silicone compositions which impart such release properties include a composition which comprises a polyorganosiloxane terminated by a hydroxyl group or a hydrolyzable group and is cured by the condensation reaction of the polyorganosiloxane with a crosslinking agent as disclosed in, for example, JP-A-47-34447 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") and a composition which comprises a polyorganosiloxane containing a silicon-bonded alkenyl group and a polyorganohydrogensiloxane containing 3 or more silicon-bonded hydrogen atoms per molecule and is cured by the addition reaction of the two kinds of siloxane polymers using a platinum catalyst as disclosed in, for example, JP-B-52-40918 (the term "JP-B" as used herein means an "examined Japanese patent publication").
However, these silicone compositions have had a disadvantage that in the case of industrially conducting continuous surface treatment of various substrates with the compositions, curing reaction should be effected at temperatures higher than 100.degree. C., generally 120.degree. C. or more, for rapid film formation. Because of this, applications of the above silicone compositions have been limited to substrates having heat resistance, and the compositions are not applicable to treatment of plastic substrates for which there is an increased desire in recent years.
On the other hand, as a technique for regulating peel strength, it has been disclosed to impart heavy-release properties by adding a tacky silicone resin ingredient to an addition reaction-type silicone composition which gives light-release peel strength as disclosed in, for example, JP-A-59-84935 and JP-A-1-215857. However, films formed from this composition are insufficient in the long-term stability of regulated peel strength, so that there has been a problem that the peel strength becomes low with the lapse of time.
As a means to eliminate the disadvantage of heat-curable silicone compositions, applications of which are limited to substrates having heat resistance, ultraviolet-curable silicone compositions have been developed.
Ultraviolet-irradiating apparatuses are being most extensively used because they are not expensive, maintenance thereof is easy, and there is little danger to the users, although they are one kind of radiation emitters. This method has advantages that curing time can normally be short and that even when the silicone compositions are applied on substrates which are damaged by heat energy, the coatings can be cured by ultraviolet irradiation without heating.
The ultraviolet-curable silicone compositions are roughly divided according to cure mechanism into the following four types:
(1) Compositions which are cured by reacting Si-Vi group (Vi means vinyl group) and Si-H group in the presence of a platinum catalyst by means of ultraviolet rays; PA0 (2) Compositions in which an acrylic-functional silicone is cured in the presence of a radical cleavage-type photocatalyst by means of ultraviolet rays (see JP-A-58-213024 and JP-A-61-293268); PA0 (3) Compositions which are cured by reacting Si-Vi group and SH group in the presence of a radical cleavage-type photocatalyst by means of ultraviolet rays (see JP-A-60- 84329); and PA0 (4) Compositions in which an epoxy-functional silicone is cured in the presence of a cation-generating catalyst by means of ultraviolet rays (see JP-A-56-166224, JP-A-58-213024, JP-A-60-47064, JP-A-1-297421, and JP-A-1-311103). PA0 (A) 100 parts by weight of an epoxy-functional polyorganosiloxane which comprises structural units represented by the formula R.sup.1 R.sup.2 SiO, wherein R.sup.1 represents hydrogen atom or a monovalent hydrocarbon group and R.sup.2 represents hydrogen atom, a monovalent hydrocarbon group, or a monovalent epoxy-functional organic group, and in which at least two of all the organic groups are a monovalent epoxy-functional organic group; PA0 (B) from 1 to 80 parts by weight of at least one copolymer for regulating peel strength, which is soluble in component (A), selected from the group consisting of PA0 (C) a catalytically effective amount of an onium salt as a photoinitiator.
Silicone compositions of type (1) above are economically disadvantageous in that the expensive catalyst should be used in a large quantity.
Compositions of type (2) cure quickly, but curing reaction should be conducted in an inert gas atmosphere because the cure is inhibited by oxygen. For this reason, they are disadvantageous in that apparatuses therefor should be specially designed and the running cost is high due to the use of an inert gas.
Compositions of type (3) have excellent curability with little curing inhibition by oxygen. However, the compositions have disadvantages that because they contain mercapto groups, they have a strong offensive odor, which is unfavorable to the workers, and that the compositions are so unstable that their shelf lives are short.
Compositions of type (4) cure by means of ultraviolet rays without suffering curing inhibition by oxygen and do not emit an offensive odor. Thus, the compositions of this type have exceedingly good properties.
In the case of the epoxy-functional silicone compositions of type (4) above, it has been disclosed to add a polyfunctional epoxy monomer or a mixture of such monomers in order to obtain various degrees of peel strength, particularly around medium-to-heavy peel strength (JP-A-60-47064). This technique, however, has had a problem that when a pressure-sensitive adhesive layer is peeled from the substrate coated with the above treating agent, peeling from the non-adhesive surface comes to be not smooth as the peeling speed increases, making a loud noise (peeling noise), and had another problem that peel strength regulation itself is difficult if delicate regulation is required, so that this technique is unsatisfactory for practical use.